Compound semiconductor devices have been developed in which a GaN layer and an AlGaN layer are formed over a substrate and the GaN layer is used as an electron transport layer. One such compound semiconductor device includes GaN-based high electron mobility transistors (HEMTs).
GaN has superior electrical characteristics. For instance, since GaN has a high saturation electron velocity and a wide band gap, it has a high breakdown voltage characteristic. GaN also has a wurtzite crystalline structure and a polarity in a <0001> direction parallel to the c axis. Furthermore, with the heterostructure of the GaN layer and the AlGaN layer, piezoelectric polarization is induced in the AlGaN layer due to the lattice distortion of both layers, generating two-dimensional electron gas (2DEG) of high concentration at an interface between the GaN layer and the AlGaN. Due to the above-described reasons, GaN has been given attention as a material of high-frequency devices and a device for electric power.
In manufacturing the GaN-based HEMT, compound semiconductor layers such as a GaN layer, an AlGaN layer and an AlN layer are formed by a metal-organic vapor phase epitaxy (MOVPE) method. However, lattice constants are different between compound semiconductor layers of different materials and also between a compound semiconductor layer and a substrate, so that distortion increases as the compound semiconductor layer becomes thicker and thus cracks are likely to form in the compound semiconductor layer. Accordingly, it is difficult to form a compound semiconductor layer of an appropriate thickness so as to provide desirable characteristics.
In consideration of these problems, there is a known structure in which a buffer layer is provided between a substrate and an electron transport layer. For instance, there is a structure in which the Al contents of AlGaN constituting a buffer layer are continuously added from the bottom. In such a structure, distortion is reduced due to the buffer layer.
However, even in the conventional GaN-based HEMT including the buffer layer mentioned above, cracks are often found in a compound semiconductor layer.
Related techniques are disclosed in International Publication No. WO 2004/066393 and Japanese Unexamined Patent Application Publication No. 2007-258406.